1. Field of the Invention
The present invention is directed to an implantable electrode system, such as a medical electrode system for stimulating living tissue as part of an implantable stimulation system.
2. Description of the Prior Art
Electrodes are known which are used in conjunction with implantable medical devices, such as devices for electrically stimulating tissue, such electrodes having a proximal end connectable to the medical device, and a distal end at which a tip electrode is located to deliver electrical stimulation pulses to surrounding tissue. The tip electrode is electrically connected to the medical device by means of a conductor which extends inside a flexible insulating sleeve from the proximal end to the distal end of the electrode. Such a device is also provided with an indifferent electrode, having a large electrode surface relative to the size of the tip electrode, and are typically provided with a second conductor, electrically insulated from the first conductor, connected to the indifferent electrode and to the medical apparatus. Electrical stimulation pulses are emitted by the medical apparatus to the tissue across the tip electrode and the indifferent electrode, and electrical signals arising in the living tissue can be sensed between the tip electrode and the indifferent electrode, and supplied to the medical apparatus for analysis.
A cardiac electrode system is disclosed in U.S. Pat. No. 4,387,717 for stimulating a cardiac heart and sensing cardiac activity. The system includes a first conductor which extends through an insulating sleeve to a tip electrode, which is placed in the heart. A ring electrode is located at a defined distance on the lead from the tip electrode. The ring electrode is connected to a second electrical conductor. The first conductor and the second conductor are connected to a pacemaker. The ring electrode serves as the indifferent electrode, and can be disposed along the lead at a distance from the tip electrode so that the indifferent electrode can be positioned either inside or outside the heart.
Particularly for sensing cardiac activity, it is important that no signals be detected by the indifferent electrode, because such signals could interfere with the interpretation of signals detected by the tip electrode. The indifferent electrode also should have a large surface, in order to reduce the current density in the region surrounding the indifferent electrode, so as to prevent needless or unwanted stimulation of body tissue around the indifferent electrode when the medical apparatus emits a stimulation pulse across the tip electrode and the indifferent electrode. The ring electrode in the known system described in U.S. Pat. No. 4,387,717 cannot be provided with such a large electrode surface, because it is stiff and therefore cannot be permitted to have a large size, because this would make the entire electrode system stiff and impossible to implant. Moreover, even if successfully implanted, a lead having a large area, rigid electrode, such as a ring electrode, may cause damage to the vascular system after implantation.
The indifferent electrode can be provided with a large surface by making the pacemaker enclosure (can) serve as the indifferent electrode. This is a known approach for pacemakers, as described in the aforementioned patent. As a result, however, the indifferent electrode may detect muscle signals from nearby muscle tissue, which would constitute noise interfering with the interpretation of cardiac signals.
Alternatively, a separate, large-area electrode could be applied outside of the heart, apart from the electrode system inside the heart, as also described in the aforementioned patent. This type of indifferent electrode, however, causes complications in implantation, and may also be difficult to affix at a suitable site without resulting in the detection of muscle signals by such an electrode.
An electrode system as disclosed in U.S. Pat. No. 3,915,174 which, in addition to a tip electrode and a ring electrode, is provided with a large-surface indifferent electrode along the length of an electrode catheter. The indifferent electrode is placed on the electrode catheter so that it lies outside of the heart after the electrode system has been implanted. This patent states that the indifferent electrode can be permitted to extend into the heart in an embodiment wherein the indifferent electrode has a size which, in principle, is coextensive with the entire electrode catheter. The indifferent electrode is formed by a cylindrical ring made of a conductive material.